The number of different antibody molecules which are normally assembled from one heavy chain or light chain V region is being investigated. The approaches to be used rely heavily on quantitative amino acid sequence analysis to determine the number of different heavy and light chains in a given antibody preparation. The solution to this problem will probably lead to a clearer understanding of how antibody diversity arises by enabling a better estimate of the number of genes required to generate a full range of antibody specificites. Bibliographic references: L.- S. Wang, J. Hora and B. Friedenson, J. Immunol. 115, 889, 1975.